Use plain symbols for eieio type descriptors (Bug#29220)

Since Emacs 26, eieio objects use a class record (with circular
references) as the type descriptor of the object record.  This causes
problems when reading back an object from a string, because the class
record is not `eq' to the canonical one (which means that read objects
don't satisfy the foo-p predicate).
* lisp/emacs-lisp/eieio.el (make-instance): As a (partial) fix, set
the record's type descriptor to a plain symbol for the type descriptor
when eieio-backward-compatibility is non-nil (the default).
* lisp/emacs-lisp/eieio-core.el (eieio--object-class): Call
eieio--class-object on the type tag when eieio-backward-compatibility
is non-nil.
(eieio-object-p): Use eieio--object-class instead of
eieio--object-class-tag.
* test/lisp/emacs-lisp/eieio-tests/eieio-test-persist.el
(eieio-test-persist-hash-and-vector)
(eieio-test-persist-interior-lists): Make into functions.
(eieio-persist-hash-and-vector-backward-compatibility)
(eieio-persist-hash-and-vector-no-backward-compatibility)
(eieio-test-persist-interior-lists-backward-compatibility)
(eieio-test-persist-interior-lists-no-backward-compatibility): New
tests which call them, eieio-backward-compatibility let-bound.

1 files changed, 3 insertions, 0 deletions

diff --git a/lisp/emacs-lisp/eieio.el b/lisp/emacs-lisp/eieio.el
index 38436d1f944..864ac2616b9 100644
--- a/lisp/emacs-lisp/eieio.el
+++ b/lisp/emacs-lisp/eieio.el
@@ -710,6 +710,9 @@ calls `initialize-instance' on that object."
     ;; Call the initialize method on the new object with the slots
     ;; that were passed down to us.
     (initialize-instance new-object slots)
+    (when eieio-backward-compatibility
+      ;; Use symbol as type descriptor, for backwards compatibility.
+      (aset new-object 0 class))
     ;; Return the created object.
     new-object))